1. Field of the Invention
The present invention relates to a display apparatus and a control method thereof.
2. Description of the Related Art
As an apparatus for generating an image, there is a digital camera using a general imaging element such as CMOS or CCD. However, the dynamic range of a general imaging element such as CMOS or CCD is narrow and is at most about 50 to 70 decibels. It is consequently difficult for such a digital camera to generate an image expressing brightness as seen by the eyes of a human. For example, when scenery with high contrast is photographed by a digital camera, a dark part is blocked up, and a bright part is blown out.
A technique for solving the problem is disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) No. 2003-46857. More specifically, in the technique disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2003-46857, a correct exposure part is extracted from each of a generally darker image (underexposed image) and a generally lighter image (overexposed image) as compared with an image with correct exposure (correct brightness). By synthesizing the extracted correct exposure parts, an image having a high dynamic range is obtained. Such a technique is called high dynamic range (HDR) image synthesis or the like. By performing the HDR image synthesis, an image relatively closer to what a human sees than an original image (an image obtained at the time of photographing) can be obtained. Generally, in an overexposed image, the gradation of a dark part is expressed finer as compared with an image of the correct exposure. In an underexposed image, the gradation of a light part is expressed finer as compared with an image of the correct exposure.
On the other hand, by alternately displaying two images having the same content and different brightnesses instead of the image synthesis as described above, the apparent dynamic range can be improved.
For example, overexposed and underexposed images each having gradation of 10 bits are alternately displayed on a display at a frame rate of a predetermined value or higher (for example, 120 fps). It can make the eyes of a human recognize an image having gradation of 11 bits.
According to the method, the HDR effect (an effect equivalent to that produced when HDR image synthesis is performed) can be obtained. The technique of improving the apparent dynamic range by alternately displaying two images having the same content and different brightnesses as described above will be called a pseudo HDR technique. When images are stored in a video output apparatus such as a camera or a PC, it is sufficient to alternately transmit the two images having the same content and different brightnesses to a display apparatus such as a television at a frame rate which is a predetermined value or higher. The video output apparatus and the display apparatus are connected to each other via, for example, a video transmission path such as an HDMI (High-Definition Multimedia Interface) cable.
Many digital televisions of recent years have a so-called frame rate conversion (FRC) function of generating video data capable of expressing smoother movement by converting the frame rate of input video data by “n” times as fast. FIGS. 11A and 11B are diagrams illustrating an example of the conventional frame rate converting process showing an example of converting video data of 60 fps to video data of 120 fps and then outputting the video of 120 fps.
In FIG. 11A, the frame rate converting process is performed by repeatedly outputting each input frame image by “r” times (r: an integer of r≧2). FIG. 11A illustrates the case where r=2. The method is effective when video data (input video signal, input video data) is a still image, and in the case where the video data is a moving image to which it is difficult to perform frame interpolation by motion compensation.
In FIG. 11B, the frame rate converting process is performed by calculating motion information (motion vector) from the two successive frame images and generating an interpolating image based on the motion information. This method is effective when the input video data includes a moving image.
By using such a method, a smoother video can be displayed on a display having a frame rate higher than that of the input video data.
However, when performing the frame rate converting process shown in FIGS. 11A and 11B on the pseudo HDR video data, a problem arises. The pseudo HDR video data is video data which successively gives two frame images having the same content and different brightnesses (for example, a frame of an overexposed image and a frame of an underexposed image). More specifically, when performing the frame rate converting process shown in FIG. 11A on the pseudo HDR video data, the frame images in the video data to be output are arranged like “light, light, dark, dark, light, . . . ”. When performing the frame rate converting process shown in FIG. 11B on the pseudo HDR video data, the frame images in the video data to be output are arranged like “light, correct, dark, correct, light, . . . ”. Consequently, intended pseudo HDR effect (the HDR effect obtained by the pseudo HDR video data) cannot be obtained. “Light” refers to a frame of an overexposed image (“+” in the diagram), “dark” refers to a frame of an underexposed image (“−” in the diagram), and “correct” refers to a frame of an image with the correct exposure.